The present invention relates to an exhaust heat recovery boiler, particularly, capable of reducing and removing a nitrogen oxide (NOx) contained in an exhaust gas.
In recent years, in order to improve an efficiency of power generation in the light of energy conservation, in addition to power generation by a gas turbine, there is a tendency of employing a combined cycle power generation plant which recovers an exhaust heat of the exhaust gas of the gas turbine so as to generate a steam and performs power generation by a steam turbine with the use the generated steam. Further, in order to improve an efficiency of power generation and a power generation output, the combined cycle power generation plant tends to be further made into a large capacity.
In the combined cycle power generation plant, an exhaust heat recovery boiler is employed to recover an exhaust heat and to generate a steam. The exhaust heat recovery boiler recovers a heat of the exhaust gas discharged from, for example, a gas turbine or diesel engine, and then, generates and supplies a driving steam for a steam turbine and a process steam hot water. Further, taking environmental protection into consideration, the exhaust heat recovery boiler includes a denitrator for reducing a harmful nitrogen oxide contained in the exhaust gas. In particular, recently, there is a tendency of providing a high performance denitrator which can remove 90% or more of the nitrogen oxide contained in the exhaust gas, in the exhaust heat recovery boiler.
A conventional exhaust heat recovery boiler will be described hereunder with reference to FIG. 22 which is a side view schematically showing the exhaust heat recovery boiler, and FIG. 23 which is a top plan view of an ammonia injection section (unit) of the exhaust heat recovery boiler.
As shown in these figures, a horizontal natural circulation type exhaust heat recovery boiler is a reheat dual pressure type boiler, which is opertively connected to, for example, a gas turbine G, diesel engine, or the like. A boiler duct 14 is provided therein with heat transfer pipes of a high pressure secondary superheater 15, a reheater 16, a high pressure primary superheater 13, a high pressure evaporator 4, a low pressure superheater 17, a high pressure economizer 18, a low pressure evaporator 19 and a low pressure economizer 20, which are located successively in the described order in the boiler duct from the upstream side to the downstream side along an exhaust gas flow direction. Further, the boiler duct 14 is provided therein with an ammonia injection section 1 and an NOx removal reactor 5, and the upper portion of the boiler is provided with a high pressure drum 6 and a low pressure drum 21. A reference numeral 2 denotes an ammonia injection section support member, a reference numeral 3 denotes a high pressure drum downcomer pipe, a reference numeral 7 denotes an ammonia injection pipe, and a reference numeral 8 denotes an ammonia injection nozzle.
Further, it is to be noted that, in the above description, the some members or units are disposed to be adaptable for high and low pressures, but in an equipment having relatively small capacity, these members or units may be utilized as one member or unit, respectively.
Next, an operation of the aforesaid exhaust heat recovery boiler will be described hereunder.
An exhaust gas flowing into the exhaust heat recovery boiler successively passes through the high pressure secondary superheater 15, the reheater 16 and the high pressure primary superheater 13, and then, is mixed with ammonia in the ammonia injection section 1. Then, the exhaust gas passes through the high pressure evaporator 4, and thereafter, nitrogen oxide contained in the exhaust gas is removed by means of the NOx removal reactor (denitration reactor or denitrator) 5 including a catalyst layer facilitating a reduction reaction. Further, the exhaust gas successively passes through the low pressure superheater 17, the high pressure economizer 18, the low pressure evaporator 19 and the low pressure economizer 20, and then, is discharged to the atmospheric air.
The ammonia injection section 1 of the exhaust heat recovery boiler is arranged on an upstream side of the high pressure evaporator 4 with respect to the exhaust gas flow direction. Further, ammonia needs to be uniformly mixed with the exhaust gas, and for this reason, the ammonia injection section 1 is arranged at a position separated from the denitration reactor 5 to some degree in a manner that the high pressure evaporator 4 is interposed between the injection section 1 and the denitration reactor 5. When passing through the high pressure evaporator 4 having many heat transfer pipes regularly arranged, the ammonia and the exhaust gas are uniformly mixed. The ammonia is oxidized at a temperature of 490.degree. C. or more, and then, a nitrogen oxide is generated. For this reason, it is not preferable to properly keep an NOx removal efficiency. Thus, a proper exhaust gas temperature is required, and then, in order to satisfy these conditions, the ammonia injection section 1 is arranged on a downstream side of the high pressure primary superheater 13 from the exhaust gas flow direction and on the upstream side of the high pressure evaporator 4, and a planned gas temperature is about 470.degree. C. In this manner, in the exhaust heat recovery boiler, a harmful nitrogen oxide contained in the exhaust gas is removed while heat exchange being made by the heat transfer pipes.
FIG. 24 is a view showing the ammonia injection section of FIG. 22 in the case of viewing from the exhaust gas flow direction.
In FIG. 24, the ammonia injection section 1 includes an ammonia injection pipe(s) 7, an ammonia injection section support member(s) 2 and a number of ammonia injection nozzles 8 formed to the ammonia injection pipe 3. The ammonia is mixed with an air in a mixer 22, and then, passes through an ammonia injection section inlet connecting pipe 23, an ammonia injection section header 24 and an ammonia injection section inlet pipe 25, and thus, flows into an ammonia injection pipe 7 supported by the ammonia injection section support member 2. The ammonia flowing into the ammonia injection pipe 7 is injected from many ammonia injection nozzles 8 provided on the ammonia injection pipe 7, and then, is mixed with an exhaust gas. These many ammonia injection nozzles 8 are vertically alternately provided on the ammonia injection pipe 7 so that the ammonia is uniformly mixed with the exhaust gas. Further, the flow rate of ammonia is controlled by means of ammonia flow control valves 26 so that the ammonia is uniformly mixed with the exhaust gas. As described above, the ammonia injection section is constructed in a manner that ammonia is uniformly injected to the overall section of exhaust gas passage in the boiler duct.
As described above, the combined cycle power generation plant has a tendency of being made into a large capacity, and for this reason, the exhaust heat recovery boiler is also made into a large size. Thus, this is a factor of causing an increase in an installation space, cost and a unit price of power generation. In order to avoid the above disadvantage, there is a need of saving a space of the exhaust heat recovery boiler and making low cost design. The conventional exhaust heat recovery boiler has a problem of requiring a large space around the ammonia injection section and a drum downcomer pipe and increasing the entire length of the boiler.
Further, the combined cycle power generation plant is made into a large capacity, thus increasing the gas turbine power output while the exhaust gas temperature rising up. Accordingly, the exhaust heat recovery boiler has also a tendency of being made into high temperature and large capacity. For this reason, in the light of environmental conservation, it is obliged for the exhaust heat recovery boiler to include a high performance denitrator.
However, in the conventional exhaust heat recovery boiler, the exhaust gas temperature rises up, and also, the temperature of the ammonia injection section rises up depending upon a system for supplying a cooling steam to a gas turbine. For this reason, there is the possibility that ammonia injection is not performed at a proper temperature. In other words, there is a problem that it is difficult to realize high NOx removal efficiency in the exhaust heat recovery boiler which is made into a high temperature and large capacity.